The use of communication systems through which to communicate data is a necessary adjunct of modern society. A wide variety of different types of communication systems have been developed and are regularly utilized to effect communication of information between sending and receiving stations positioned at the separate, spaced-apart locations. This communication between distant stations may be carried via a wire, or more typically, a series of wires forming a communications network. Such a network may include a multitude of switches so that any station connected with the network may communicate with any other connected station. In addition to wireline systems, wireless radio frequency signals may also carry communication information over part or all of the communication path. Still other forms of information transport include microwave, infrared, and optical signals.
Improvements to existing types of communication systems, as well as implementation of new types of communication systems, have been made possible as a result of advancements in communication technologies. The advances include not only more efficient and effective hardware, but better techniques for information processing and transmission as well. The resulting improvements include better transmission quality, lower error rates, fewer losses of connection, and increased network capacity. Radio communication systems are exemplary of communication systems that have benefited from these advancements in communication technologies. Modern radio networks transmit a much greater quantity of information, and do so more faithfully, than their less sophisticated predecessors.
A radio communication system, of course, inherently permits an increase in communication mobility in contrast to communications effectuated through the use of conventional, wireline communication systems. Increased communication mobility is provided as the communication channels between the sending and receiving stations of the radio communication systems are defined upon radio links formed therebetween. Typically, users subscribing to service through a wireless radio communication network carry their own uniquely identifiable mobile station. The mobile stations communicate with a fixed base station, which is, in turn, connected in some way to the rest of the network. The channel from the base station to the mobile station is often referred to as the forward channel, and the reverse channel carries transmissions from the mobile back to the base station. Note that the radio communication channels do not require fixed connections between the sending and receiving stations for their formation, but rather may be repeatedly established and broken on an ad hoc basis.
One type of radio communication system that has achieved high levels of usage is the cellular communication system. In a cellular system, the network-covered geographic area is divided into relatively small sections, or cells, each having at least one antenna connected to a base station. Mobile stations are able to communicate with the base station of the cell in which they are currently located and base stations of the neighboring cells, though generally not far beyond that. In this way, channels can be re-used by many cells, so long as the cells sharing a given channel are not adjacent (or too close) to each other. This arrangement permits the system to handle many more calls at one time than if each call had to be assigned channels unique throughout a large area. In addition, various techniques have been devised for sharing available bandwidths among a group of callers, even if they are in the same cell. Time-division, multipleaccess (TDMA), for example, divides the available bandwidth into time slots, with one or more time slots being assigned to each communication. TDMA takes advantage of the fact that few ongoing communications require a channel one-hundred percent of the time. Another scheme, one that will be further described along with an embodiment of the present invention, is code division multiple access (CDMA) in which multiple communications can be processed and sent out at the same time, but received and processed only by a target mobile station in possession of necessary information about how the signal was encoded before transmission. CDMA in this way also makes it difficult for calls to be intercepted—a great advantage to many subscribers.
When advances in technology occur, changes to the network components may be necessary. When a network infrastructure of a cellular communication system is to be upgraded, the network infrastructure of such communication system is preferably altered in a manner as to effect the upgrade while also permitting existing mobile stations, operable pursuant to the previous communication-system requirements to continue operation in the upgraded or revised system. Although equipment owned by the network operator may be changed out promptly when the upgrade is initiated, subscriber-owned mobile stations may continue to be used for months or even years before being replaced. The subscribers will still want to use them. The quality of an upgrade or revision that allows old as well as new equipment to be used with the upgraded network is called backward compatibility. Were it not present, hundreds of subscribers might suddenly find themselves unable to place calls without buying a new telephone. Backward compatibility is, therefore, an important feature.
In order to ensure backward compatibility, the upgraded system must be carefully constructed in order to provide for the communication using both old and new mobile stations. Many, if not all, facets of operation of the communication system must be considered to insure the backward compatibility of the devices intended to be operable therein.
Synchronization is one such facet. Synchronization operations performed between the network infrastructure and the mobile station of the cellular communication system are required prior to effectuation of voice and data traffic communications between the network infrastructure and a mobile station. Synchronization of a mobile station with a network infrastructure insures that the timing of operations of the network infrastructure and the mobile station is necessary in CDMA systems for the mobile station to be able to detect and process a received signal traffic (that is, encoded voice or data information). In CDMA systems, a synchronization message (sync message) is sent out (broadcast) continuously from base stations within the network over a synchronization channel (sync channel). The sync message contains the identity of the base station, system time, and other information so that the mobile stations in the vicinity receive the information and can perform sync-dependent processing of other transmitted signals. Without such synchronization, multiple-access schemes such as CDMA would fail utterly.
Backward-compatibility of synchronization operations must therefore be provided when a cellular communication system is upgraded, or otherwise revised, so that both mobile stations constructed to be operable pursuant to the upgraded communication system and mobile stations constructed to be operable pursuant to the pre-existing cellular communication system can be synchronized to the network infrastructure of the upgraded, or revised communication system. As noted above, cellular communication systems have been constructed pursuant to various multiple-access schemes such as CDMA. An exemplary implementation of a CDMA cellular communication system is referred to as an IS-95 (interim standard-1995) system. Such a system is constructed to be operable pursuant to operational requirements set forth in the TIA/EIA IS-95 standard promulgated by the Telecommunications Industry Association (TIA) and the Electronic Industries Alliance (EIA), which are industry regulatory groups. IS-95 communication systems have, for several years, been installed and commercially implemented in various geographical regions. Mobile stations constructed according to the IS-95 standard should work satisfactorily in these regions regardless of their manufacturer.
Proposals have been set forth, however, for a revised set of CDMA communication-system standards that are commonly referred to as TIA/EIA IS-2000. The IS-2000 standard is intended to specify a CDMA spread-spectrum radio interface that, among other things, meets the requirements for third generation (3G) wireless communications systems. Although backward compatibility was intended, it has been reported that the IS-2000 sync message is posing difficulty for some IS-95 mobile stations. The IS-2000 standard pertaining to synchronization operations differs with the synchronization operations set forth in the pre-existing IS-95 system. In particular, a synchronization message, herein referred to an IS-2000 sync channel message, includes an additional eleven-bit field (EXT_CDMA_FREQ) together with five bits of zero-padding relative to the synchronization message used in an IS-95 system. The additional field and zero-padding bits causes the message transmission of an IS-2000 sync channel message to extend into an additional 26.66 ms sync channel frame compared to the pre-IS-2000 message. Some mobile stations constructed to be operable pursuant to the IS-95 system are unable to properly detect and act upon the increased-length sync channel message, as it is presently defined.
An alternate synchronization message scheme is therefore required for the IS-2000 or other system that is better able to be backwardly-compatible to pre-existing system types, while still meeting the requirements of 3G and other future systems.
It is in light of this background information related to the radio communication systems that the significant improvements of the present invention have evolved.